Star-forming cloud complexes in the central molecular zone of NGC 253

Abstract

We report 350 and 230GHz observations of molecular gas and dust in the starburst nucleus of NGC253 at 20-40pc (1″-2″) resolution. The data contain CO(3-2), HCN(4-3), CO(2-1), 13CO(2-1), C18O(2-1), and continuum at 0.87mm and 1.3mm toward the central kiloparsec. The CO(2-1) size of the galaxy's central molecular zone (CMZ) is measured to be about 300pc×100pc at the half-maximum of intensity. Five clumps of dense and warm gas stand out in the CMZ at arcsecond resolution, and they are associated with compact radio sources due to recent massive star formation. They contribute one-third of the CO emission in the central 300pc and have 12CO peak brightness temperatures around 50K, molecular gas column densities on the order of 104 M pc-2, gas masses on the order of 107 M in the size scale of 20pc, volume-averaged gas densities of cm-3, and high HCN-to-CO ratios suggestive of higher fractions of dense gas than in the surrounding environment. It is suggested that these are natal molecular cloud complexes of massive star formation. The CMZ of NGC253 is also compared with that of our Galaxy in CO(2-1) at the same 20pc resolution. Their overall gas distributions are strikingly similar. The five molecular cloud complexes appear to be akin to such molecular complexes as SgrA, SgrB2, SgrC, and the l = 13 cloud in the Galactic center. On the other hand, the starburst CMZ in NGC253 has higher temperatures and higher surface (and presumably volume) densities than its non-starburst cousin. © 2011. The American Astronomical Society. All rights reserved..